selftests/seccomp: add support for s390

This adds support for s390 to the seccomp selftests. Some improvements
were made to enhance the accuracy of failure reporting, and additional
tests were added to validate assumptions about the currently traced
syscall. Also adds early asserts for running on older kernels to avoid
noise when the seccomp syscall is not implemented.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>